Tracking Southern China's Forest Growth from Space

Forest canopy height reflects the vertical structure of forests and gives indications on the growth capacity of trees and the level of forest biomass. Despite an increasing availability of global canopy height maps, there is a lack of maps reflecting temporal dynamics, which is required to understand forest carbon sink capacity. This study uses publicly available global maps and Landsat data to construct a long time series of canopy height maps in southern China (1986 to 2019). Our predictions are in the same range as National Forest Inventory data, both spatially and temporally. The dataset shows clear signs of tree growth, with small trees dominating the first period, which grow taller over the 3 decades. In numbers, the mean canopy height of the forests in southern China increased from 6.4 ± 3.99 m in 1986 to 10.3 ± 5.54 m in 2019, which is an increase of 61.25%. Secondary forests have an overall higher canopy height (17.8 ± 2.12 m) as compared to plantations (14.6 ± 2.05 m), but plantations grow faster (0.20 ± 0.08 m/ year) as compared to secondary forests (0.13 ± 0.08 m/year). A driver analysis shows that management is the dominating factor for the canopy height increase, while natural factors play a minor role. Our study reaffirms the human made surge in forest areas in China, quantifies their canopy height dynamics, and demonstrates that spectral data can indeed be used to track canopy height changes at scale.